<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
      #triangle-up {
        width: 0;
        height: 0;
        border-top: 100px solid transparent;
        border-left: 100px solid transparent;
        border-right: 100px solid transparent;
        border-bottom: 100px solid red;
      }
    </style>
</head>
<body>
  <div id="triangle-up"></div>
</body>
</html>
